In recent years, in such manufacturing processes of, for example, a semiconductor, a liquid crystal substrate, or a color filter, the processing time thereof is shortened, and one time exposure to a work piece having a large area, etc. is advanced by using an ultraviolet light source with large input electric power. With such advances, emission of light with higher brightness is required for a high pressure discharge lamp, which is an ultraviolet light source. However, when input electric power to such a high pressure discharge lamp is simply increased, a load to electrodes arranged inside a discharge vessel thereof increases, so that there is a problem that the high pressure discharge lamp is blackened due to material evaporated from the electrodes, whereby a life span thereof is shortened.
As to the technology of a light source apparatus, U.S. Patent Application Publication No. 2007/0228300, which was recently published, teaches the technology of a LDLS (Laser-Driven Light Source) in which a light with a high brightness is emitted by supplying energy by laser to a gas, which is in an ionization state in a discharge vessel space. FIG. 6 shows a structural example of such a light source apparatus, for explaining the laser driven light source technology disclosed in U.S. Patent Application No. 2007/0228300. A brief description of the structure thereof will be given below, referring to the figure. A laser beam is emitted from a laser oscillator 804, so as to reach a collimator 812 through a fiber cable 808. The laser beam 816, formed into parallel light at the collimator 812, is condensed at an optical lens 820, thereby forming a laser beam 824 having a small diameter, and directed at an area 830 in a chamber 828 corresponding to an discharge vessel of a discharge lamp, in which a plasma 832 exists. And a medium (gas), which is ionized between a pair of electrodes 840, is irradiated with the laser so as to emit a light with a high brightness.
According to the laser driven light source technology, high brightness light emission is deemed to be maintained by irradiation of laser. However, even though the laser driven light source technology is applied to a discharge lamp in which a pair of electrodes is arranged in an discharge vessel thereof, as in the conventional discharge lamp having electrodes between which voltage for maintaining discharge is impressed, deposits consisting of elements of the enclosure material in the discharge vessel, which mainly consist of spattered electrode material, increase gradually on the inner face of the discharge vessel, as the lighting time progresses, so that it is impossible to prevent a blackening from becoming visible. Moreover, even in a non-electrode type discharge lamp in which electrodes are not arranged in an discharge vessel thereof, depending on the cooling conditions of an discharge vessel wall etc., vapor from aggregation substances of, for example, sulfur, mercury, etc., which are contents thereof, adheres to the discharge vessel wall, so that a phenomenon of opacity of the discharge vessel, which is equivalent to the blackening of the electrode discharge lamp, may happen.
Thus, due to the blackening of the discharge vessel, or due to the vapor from aggregation substances of, for example, sulfur, mercury, etc., which are contents thereof, adhering to the discharge vessel wall, it turns out that, it becomes hard to start discharge in the discharge vessel thereof in the case of a discharge-starting unit for which a laser is used, or, discharge may not be stably maintained after the discharge begins in the discharge vessel in the case of a discharge-starting means for which an electrode is used, even though a laser is emitted thereto to maintain discharge.